1. Field of the Invention
The present invention relates to spectroscopy. More particularly, the present invention relates to the spectral characterization of crude oil using time-resolved laser-induced fluorescence spectroscopy.
2. Description of the Related Art
Characterization of petroleum oil is customarily made using a number of analytical methods that investigate its physical and chemical properties(See Ali. xe2x80x9cFull Range Crudes, Analytical Methodology of.xe2x80x9d in Encyclopedia of Analytical Chemistry. R. A. Meyers (Ed.) pp. 6709-6726. John Wiley and Sons Ltd, Chichester, 2000)). These methods, which have been standardized comprehensively by ASTM, aim at identifying a number of characteristics such as density, thermal stability, heavy-metal contents, types and quantities of the hydrocarbon groups, types and quantities of the aliphatic and aromatic compounds, etc., which collectively can provide complete sets of data useful in characterizing the petroleum oil. However, because all of these analytical methods require sample preparation, they cannot be used in situations where instant and/or remote characterization is needed. In such cases the laser-induced fluorescence methods would be more suitable. These latter methods give information about the oils by investigating their broad emission spectra when they are excited at specific wavelengths.
The resulting broad emission spectra are due to hundreds of different compounds comprising the oils, which fluoresce with effective lifetimes ranging from a few picoseconds to a few tens of nanoseconds. To get laser-induced fluorescence spectra that are as useful as possible in the characterization process, the excitation wavelength should be in the UV region so that the produced spectra will include contributions from the light aromatic compounds, which usually provide important features along the short wavelength end of the spectra.
The usefulness of these laser-induced fluorescence methods as characterization methods depends largely on the type of the technique employed. There are several of these techniques whose characterization abilities range from merely detecting the presence of oil, as a pollutant for example, to actually distinguishing the different types of oils from each other(See Eastwood. Modern Fluorescence Spectroscopy, V 4 Wehry, Plenum and references therein (1981)). The simplest of these methods is an approved method by both the American Society of testing and materials (ASTM) (See ASTM Book of Standards (1978), p. 720, D3650-78) and the US Coast Guard(See Oil Spill Identification System, Chemistry Branch, U.S. Coast Guard RandD Center, Report No. DOT-CG-D-52-77 (June 1977)3, and it relies on recording the wavelength-resolved fluorescence spectra while exciting the oil with a single UV radiation at 254 nm.
This method can, in principle, be applied in remote sensing, but the information derived from it will allow the distinguishing between only the broad classes of oils, e.g., between light refined oil, crude oil, and heavy residual oil, and not between different grades of oils belonging to the same broad class, e.g., between Light crude oil and Medium crude oil. To do so other fluorescence techniques such as the synchronous scan fluorescence spectroscopy, contour(total luminescence) spectroscopy, or time-resolved fluorescence spectroscopy should be employed.
The synchronous scan fluorescence spectroscopy technique produces spectra resulting from scanning both the excitation wavelength and the detection wavelength with a fixed wavelength separation. By using this technique(See Lloyd. xe2x80x9cThe nature and evidential value of the luminescence of automobile engine oils and related materials. Part I. Synchronous excitation of fluorescence emission.xe2x80x9d Journal of Forensic Science Society, vol. 11, pp. 83-94 (1971); Lloyd. xe2x80x9cThe nature and evidential value of the luminescence of automobile engine oils and related materials. Part III. Separated Luminescence.xe2x80x9d Journal of Forensic Science Society. Vol. 11, pp. 235-253 (1971); Lloyd. xe2x80x9cThe nature and evidential value of the luminescence of automobile engine oils and related materials. Part II. Aggregate Luminescence.xe2x80x9d Journal of Forensic Science Society. Vol. 11, pp. 153-170 (1971); Lloyd xe2x80x9cPartly Quenched, Synchronously Excited Fluorescence Emission Spectra in the Characterization of Complex Mixtures.xe2x80x9d Analyst vol. 99, pp. 729-738 (1974); and Vo-Dinh, et al. xe2x80x9cPolynuclear Aromatic Hydrocarbonsxe2x80x9d, 3rd International Symposium of Chemical Biologyxe2x80x94Carcinogens and Mutagens, p. 111 (1978)) it is possible to distinguish oils belonging to the same broad class from each other, e.g., Light crude oil from Heavy crude oil, but the distinguishing ability is still not adequate enough to discriminate between crude oils of closer grades, such as Medium crude oil and Heavy crude oil(See Shen, et al. xe2x80x9cIdentification of spilled crude oils from similar Origins.xe2x80x9d Arabian Journal of Science and Engineering, vol. 10, p. 63 (1984)). In addition, this technique cannot be practically used in remote sensing since it is not easy to tune a high-intensity laser over a wide range of excitation wavelengths.
The contour (total luminescence) spectroscopy technique(See Hornig, Proceedings, Pattern Recognition Applied to Oil Identification, Coronado, Calif. (1976); Warner et al. xe2x80x9cAnalysis of Multicomponent Fluorescence Data.xe2x80x9d Analytical Chemistry, vol. 49, p. 564-573 (1977); and Giering et al. xe2x80x9cTotal Luminescence Spectroscopy, A powerful technique for mixture analysis.xe2x80x9d American Laboratory, vol. 9 No. 11, pp. 113-123 (1977)) is another technique that can be used for the purpose of crude oil characterization. It produces contour diagrams of oils that are constructed out of many emission spectra each of which is excited at a different wavelength.
This method has a good distinguishing ability between oils belonging to the same broad class, but it is not a method that can be applied practically in remote sensing studies either for the same reason as that mentioned above for the synchronous scan fluorescence technique.
The laser-induced fluorescence technique that promises a good distinguishing ability and, at the same time, a practical remote sensing application is the time-resolved laser-induced fluorescence, technique. The suggestion of this technique as a tool for oil characterization was made as early as 1971 by Fantasia et al(See J. F. Fantasia, T. M. Hard, and H. C. Ingrao. Report No. DOT-TSC-USCG-71-7, Transportation Systems Center, Dept. of Transportation, Cambridge, Mass. (1971) and J. F. Fantasia and H. C. Ingrao. Proc. Of the 9th Intern. Symp. On Remote sensing of the environment, Ann Arbor, Mich., Apr. 15-19, 1974, Paper 10700-1-X, 1711-1745)), who recommend the use of lifetime measurements as an additional tool for crude oil characterization.
Immediately thereafter, Measures et al(See Measures et al., xe2x80x9cLaser Induced Fluorescent Decay Spectra, A New Form of Environmental Signature.xe2x80x9d Optical Engineering, vol. 13 pp. 494-501 (1974) and Measures et al. xe2x80x9cLaser Induced Spectral Signatures of Relevance to Environmental Sensing.xe2x80x9d Canadian Journal of Remote Sensing, vol. 1, No. 2, pp. 95-102 (1975)) conducted experiments to study the variation of the fluorescence decay time as a function of wavelength across the emission profile for a variety of materials.
They concluded that, in the case of a complex mixture of molecules, this variation could be used to discriminate between very similar substances, i.e., it could be used as a tool for true fingerprinting. Camagni et al(See Camagni et al. xe2x80x9cDiagnostics of Oil Pollution by Laser Induced Fluorescence.xe2x80x9d IEEE Transactions on Geoscience and Remote Sensing, vol. GE-26, No. 1, pp. 22-26 (1988) and Camagni et al. xe2x80x9cFluorescence Response of Mineral Oils: Spectral Yield vs Absorption and Decay Time.xe2x80x9d Applied Optics, vol. 30, No. 1, pp. 26-35 (1991)) did one of the early applications of this technique in remote sensing in the mid 1980""s. They used a pulsed laser of 4-ns pulse width to excite the fluorescence spectra of crude oils and then aimed at drawing a relation between the temporal decay behaviors measured at different wavelengths to the type of commercial crude oils they studied. Instead of directly de-convolving the instrumental response from the resulting temporal decay curves, which is not usually feasible in remote sensing, they resorted to submitting their data to some regression analysis so as to check the existence of a good deterministic power-law correlation among the different decay curves, which can be considered as the convolution of the instrumental response. Using the following relationship:
Yxcex(t)=axc2x7[Xxcex*(t)]b
(where Yxcex(t) represents the observed time response of oil Y at wavelength xcex, [Xxcex*(t)] is the observed time response of an arbitrary sample of known exponential behavior, and a and b are the regression parameters), they found that the two quantities characterizing the individual samples namely; average decay time xcfx84xcex, and relative efficiency xcfx81xcex, become directly related to the regression parameters a and b. Their work showed that these two, quantities could be used meaningfully in the identification off various commercial crude oils.
The work of Camagni et al. represents an application of the time-resolved fluorescence technique to identify crude oils remotely by looking at both the temporal and the spectral characteristics of oils. There are other workers such as Diebel eti al. (See D. Diebel, T. Hengstermann, and R. Reuter, in Remote sensing of pollution of the sea, edited by R. Reuter and R. H. Gillot (Oldenborg: Commission of the European Communities Joint Research Center, ISPRA Establishment and BIS Universitat), SPI 87.46, pp. 266-280 and Diebel et al. Proceedings of an international meeting of the institute of petroleum, London May 1988 (Chichester: John Wiley and Sons), pp. 127-142., Koechler et al(See Koechler et al. Proceedings of S.P.I.E. Conference on Lidar for remote sensing, Berlin, Federal Republic of Germany, 1992 (Bellingham, Wash.: International Society for Optical Engineering), pp. 93-107)), Quinn et al. (See Quinn et al. xe2x80x9cMeasurement and Analysis Procedures for Remote Identification of Oil Spills Using a Laser Fluorosensor. Journal of International Remote Sensing, vol. 15 pp. 2637-2658 (1994)) and others who also applied similar techniques for the same purposes. The method of ""810 does not utilize the time-resolved fluorescence technique.
The availability of streak cameras and other gated CCD devices provides the user of the time-resolved fluorescence technique with the advantage of having instant images of the overall fluorescence intensities as functions of both time and wavelength to be further processed. These digitized images, however, have the instrument response embedded in them, and hence, they still need further de-convolution procedures to extract the wavelength-dependent parameters characterizing the oils, such as the average decay times and the relative efficiencies of the observed fluorescence. This additional de-convolution step is problematic in practice, especially if the identification of oils is to be done remotely, and it also adds to the uncertainty of the results especially when some sort of approximation is needed to determine it.
It would be desirable to provide a simple, readily followed method for characterization of petroleum oils which avoids the pitfalls and complexities of prior characterization methods discussed above.
U.S. Pat. No. 5,656,810, issued Aug. 12, 1997, to Alfano et al., describes a method for evaluating a crude oil sample using spectral differences in the luminescence, excitation, light scattering, and absorption spectra in the near UV, visible, and near IR regions for various crude oils. In one preferred embodiment, the method comprises illuminating an oil sample with light of a suitable excitation wavelength, measuring the resultant fluorescence therefrom, and comparing the resultant fluorescence to appropriate standards derived from known components of crude oil.
The absorption spectra from 190 nm to 2000 nm for the samples show differences. The deasphalted oil sample appears to show less absorption and saturates in the visible spectrum below about 600 nm. Specific examples use excitation wavelengths of 300 nm, 350 nm, 400 nm, and 450 nm.
U.S. Pat. No. 6,140,048, issued Oct. 31, 2000, to Mxc3xcller et al. describes a system and method for distinguishing at least two types of molecule groups by time resolved fluorescence measurements. Light sources used for exciting the molecules have an emission wavelength of about 600 nm to 900 nm. The method of the ""048 patent does not employ the shapes of the time-resolved spectra.
U.S. Pat. No. 5,565,982, issued Oct. 15, 1996, to Lee et al., describes an apparatus and a method for time resolved spectroscopy using pseudo-random modulated diode lasers.
U.S. Pat. No. 5,049,738, issued Jun. 4, 1991, to Gergely, et al., describes a method and apparatus for precise oil correlation using oil-filled fluid inclusions to form signature plots of fluorescence excitation versus emission versus intensity.
U.S. Pat. No. 5,780,850, issued Jul. 14, 1998, to DeLaune, et al., describes a method for evaluating the ∘API gravity of a sample of underground formation including measuring the emission fluorescence of a solvated sample at a fixed excitation wavelength with measurements of emission intensities at two points and characterizing the oil by the ratio of two emission intensities obtained at a fixed excitation wavelength, determining the yield, and applying regression analysis to a data base of oils to obtain an equation which results in an algorithm value, and interpreting the value of the algorithm to give a value for ∘API gravity and estimate in-situ oil concentration.
U.S. Pat. No. 6,268,603, issued Jul. 31, 2001, to Mullins et al., describes methods and apparatuses for investigating formation surrounding a borehole by acquiring a fluorescent signal over the borehole and analyzing the signal to detect the presence of crude oil and to characterize the crude oil.
The present invention deals with oil identification and/or characterization using the time-resolved fluorescence technique also, but it employs a different way of presenting the temporal spectral characteristics of oils. Instead of measuring lifetimes as functions of wavelength, it measures the variations in the spectral profiles of the emitted fluorescence spectra as functions of time(See Hegazi et al. xe2x80x9cNew Approach for Spectral Characterization of Crude Oil Using Time-Resolved Fluorescence Spectra.xe2x80x9d Applied Spectroscopy, vol. 52 pp. 202-207 (2001), hereby incorporated by reference.
The technique depends on producing contour diagrams of the fluorescence intensities as functions of time and wavelength simultaneously, which resemble the sort of digital image produced by streak cameras and other gated CCD devices. The difference between them, however, is that these contour diagrams are constructed out of the time-resolved spectra that have been normalized in intensity at a certain particular emission wavelength, and therefore they will show contours representing the xe2x80x9cshapesxe2x80x9d of the time-resolved spectra alone with no consideration given to their xe2x80x9crelative intensitiesxe2x80x9d as in the case of the streak, cameras and the other gated CCD devices. In other words, these diagrams will not have the intensity of the laser-pulse response imbedded in them, and as such they will have a better chance of showing certain detailed features that can be used to distinguish oils from one another.
The method of the present invention depends on the monitoring of the variations occurring in the shapes of the normalized time-resolved fluorescence spectra with respect to time. Contour diagrams constructed from these spectra are portrayed as fingerprints useful to identify oils without the need for further mathematical analysis, such as deducing the effective lifetimes.
The present invention makes it possible to distinguish the, different grades of crude oils, even if the excitation wavelength is not near 254 nm. An excitation wavelength at 355 nm, for example, can produce fingerprints capable of distinguishing blended crude oils of different commercial grades.
The fingerprints are also useful in the monitoring of the degradation of lubricant and transformer oils. And in another immediate application of the invention the ∘API gravity of crude oils can be directly estimated by analyzing the shapes of the time-resolved fluorescence spectra. (See Hegazi et al., above). (∘API is the American Petroleum Institute gravity, which is one of the schemes by which crude oils are classified and is based on specific gravity values of the 250 to 275 C (1 atm) and the 275 to 300 C, (40 mm) distillation fractions.)
None of the above inventions and patents, taken either singularly or in combination, is seen to describe the instant invention as claimed. Thus a method for characterization of petroleum oils using normalized time-resolved fluorescence solving the aforementioned problems is desired.
When a crude oil sample is irradiated with a pulsed UV laser radiation it emits a bluish white light (referred to as fluorescence) that lasts for 20-50 ns after the end of the excitation pulse. If the laser pulse width is of the order of 10 ns then considerable overlap occurs between the temporal responses of the excitation and the fluorescence pulses. This invention is a method of obtaining useful temporal-spectral information about the crude oil without resorting to de-convolving the temporal response of the laser pulse from the resulting fluorescence signal. This is done by 1) measuring the excited fluorescence spectra at narrow time gates(TG""s) of 2 or 5 ns within the temporal overlap region, 2) normalizing these time-resolved fluorescence spectra at a particular emission wavelength to highlight the shapes of their profiles, and then 3) comparing the changes in these shapes as function of time and wavelength simultaneously either by constructing contour diagrams (which present some sort of visual fingerprint) or by measuring areas under the curves of particular, emission regions.
The results depend on the shape of the laser pulse but not on its intensity, and therefore they would be universal if a laser, pulse of particular standard shape and a standard detection response were to be employed. To produce fingerprints of high. distinctive features, the wavelength of the excitation laser pulse should be at 266 nm or shorter so that the fluorescence spectra will include contributions from the light aromatic compounds. However, the method can still produce adequate fingerprints, but of less distinctive features, if longer wavelengths (i.e., 355 nm) are used, instead. In fact the ability to use the 355 nm wavelength for fingerprinting crude oils is one of the advantages of this method over the other time-resolved fluorescence methods.
Besides fingerprinting crude oils and their thermal distilled fractions, the invention can also be used to monitor the degradation of mineral oils, such as lubricants and transformer oils.
Accordingly, it is a principal object of the invention to provide a method for the characterization and fingerprinting of petroleum oils and other complex mixtures.
It is another object of the invention to provide a method as above based on time-resolved, laser-induced fluorescence spectroscopy.
It is a further object of the invention to provide a method as above which provides fingerprints of crude oils and other complex, mixtures without resorting to any kind of approximation.
Still another object of the invention is to provide a method as above capable of distinguishing between closely similar crude oils of the same grade.
Yet another object of the invention is to provide a method as above capable of estimating the ∘API gravity value of crude oils.
Still another object of the present invention as above capable of monitoring the degradation of mineral oils used in lubrication and transformers.
Yet another object of the invention is to provide a method as above depending on exciting the wavelength-resolved fluorescence of samples with ultraviolet pulsed laser radiation, measuring them at specific time gates within the temporal response of the excitation laser pulse, and comparing them in terms of their shapes alone, i.e., fingerprints.
It is an object of the invention to provide improved methods for the purposes described which are inexpensive, dependable and fully effective in accomplishing their intended purposes.